Rotational motion compensated seabed seismic sensors and methods of use in seabed seismic data acquisition

ABSTRACT

Apparatus and methods for acquiring seismic data using a seabed seismic data cable positioned on a seabed are described, including correcting for the effect of one or more sensor non-linear motions, which improves accuracy of seismic data. One or multiple non-linear movements of the sensor may be corrected for. It is emphasized that this abstract is provided to comply with the rules requiring an abstract, which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/669,564, filed Apr. 8, 2005,incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to the field of seismic dataacquisition, and particularly to sensors and cables adapted to lie on aseabed and methods of using same to acquire seabed seismic data.

2. Related Art

Marine seismic exploration investigates and maps the structure andcharacter of subsurface geological formations underlying a body ofwater. In so-called seabed seismic, a cable containing seismic receiversis deployed onto the seabed from a surface vessel. In theory, it is nowpossible to use so-called multi-component data acquisition, wheremulti-component refers to compressional wave and shear wave componentseismic surveying. However, in practice, results may be sub-optimal dueto inadequate coupling of the sensors to the seabed, and rotation ofsensor housings along one or more axis.

It is evident that there is a need in the art for improvement in seabedseismic data acquisition.

SUMMARY OF THE INVENTION

In accordance with the present invention, seabed seismic sensors, cablesincluding same, and methods of using same in seabed seismic dataacquisition are described which compensate for rotational, rollingand/or pivoting motion of the sensor unit, whether due to currents, orthe seismic waves themselves, or both. The rotational, rolling, and/orpivoting motion may be referred to herein as non-linear motion.

A first aspect of the invention is an apparatus comprising a seabedseismic sensor, the sensor comprising at least one linear motion seismicsensing unit, and means for sensing one or more non-linear motions ofthe seabed seismic sensor. The non-linear motion may be rotation,rolling, or pivoting, and may be about any axis of symmetry, or any axissubstantially parallel to an axis of symmetry of the seabed seismicsensor.

Apparatus in accordance with the invention may include a plurality oflinear motion seismic sensor units, which may be positioned inside thecable body, or may be placed in sensor housings, the housings fastenedto the cable. As used herein the phrase “seabed seismic sensor” includesany housing used. In either case the linear motion seismic sensor unitsmay be spaced along the cable in any fashion. Seabed seismic sensors areoften of a shape that attempts to minimize or prevent rotational,rolling and pivoting of the seabed seismic sensor for example. This isnot always perfectly achieved. The seabed seismic sensors and methods ofthe invention compensate for distorted signals associated with one ormore non-linear motions of the seabed seismic sensor which are recordedby the linear motion seismic sensing units if the rotation, pivoting, orrolling is about a line parallel with the longitudinal axis of symmetryof the seabed cable. In some embodiments, when distorted signalsassociated with non-linear motion about an axis substantially parallelto the longitudinal axis of symmetry of the seabed cable are to becorrected for, the shape and perhaps other characteristics of the seabedseismic sensor may be selected so that the non-linear motion of theseabed seismic sensor about an axis substantially parallel to thelongitudinal axis of symmetry of the seabed cable may not be limited, incontrast to previous designs. Rather, the non-linear motion of theseabed seismic sensor about an axis substantially parallel to thelongitudinal axis of symmetry may be used to advantage and measured bymeans for sensing non-linear motion in correcting for transfer of thehorizontal particle displacement of the sea-bottom to a horizontalmovement of the seabed seismic sensor. The presence of the seabedseismic sensor itself will disturb the local motion of the Earth, butthis problem may be mitigated by selecting the specific density of theseabed seismic sensor to be equal to or nearly equal to that of theEarth in the vicinity of the seabed seismic sensor. Apparatus of theinvention include those wherein the means for sensing non-linear motionis selected from one or more non-linear motion sensors, one or morelinear motion sensors in conjunction with three linear motion sensorsadapted to sense direction of the seismic wave front, and one or moresensors capable of sensing both non-linear and linear motion. The meansfor sensing non-linear motion may be selected from sensors capable ofmeasuring acceleration in one degree of non-linear motion freedom andsensors capable of measuring accelerations in two or more degrees ofnon-linear motion freedom. The means for sensing non-linear motion mayinclude a linear motion sensor, which may be selected from sensorscapable of measuring acceleration in one degree of linear freedom andsensors capable of measuring accelerations in two or more degrees oflinear freedom. Apparatus of the invention include those wherein themeans for sensing non-linear motion about at least one axis comprisesone or more micro-machined-electro-mechanical sensors, and those whereinthe means for sensing non-linear motion comprises a three-componentlinear motion micro-machined-electro-mechanical sensor. Thethree-component linear motion micro-machined-electro-mechanical sensormay be positioned in a center of a housing of the seabed seismic sensor.

A second aspect of the invention are methods comprising:

-   -   (a) acquiring seismic data using a seabed seismic sensor on a        seabed;    -   (b) sensing non-linear motion of the seabed seismic sensor; and    -   (c) correcting the seismic data for the non-linear motion of the        seabed seismic sensor.

Methods of the invention include those wherein horizontal motion of theEarth, sea currents, and other substantially horizontal forces areconverted into non-linear motion of the seabed seismic sensor, therebydistorting the horizontal motion at the seabed seismic sensor locationwith respect to the motion at the Earth-to-sensor-contact.

In addition to measuring seabed seismic sensor non-linear motion, seabedseismic sensor non-linear motion may be modeled under the expectedenvironmental and seabed conditions and data stored in a usable fashion,such as in a database or data warehouse until needed, along withmeasured data from previous surveys. Either modeled or previous surveynon-linear motion records or both may be used for quality controlpurposes, as well as for correcting the multi-component measurements.The correction of multi-component measurements may be more complex insome environments, for example when a cylindrical sensor housing (or aportion thereof) is not resting on a flat sea floor. In theseenvironments, certain embodiments of the invention may perform acorrection based on modeling or based on laboratory measurements ofseabed seismic sensor response to non-linear motion.

In certain embodiments of the invention the DC component of gravity maybe measured and used for decomposition of signals from fixed axis seabedseismic sensors into vertical and horizontal components. This may beperformed by using MEMS accelerometers measuring the DC component, or bya combination of geophones and inclinometers. One benefit of thesemethods is that they will allow for a more accurate compensation of thenon-linear movement. Without the accurate knowledge of orientation ofthe seabed seismic sensor (for example in embodiments wherein gimbaledgeophones are used if the sensor unit rotates, or fixed geophoneswithout inclinometers), there would be an uncertainty in the directionalong which the seismic signal is recorded, which will be detrimental tothe data quality in itself, and which in addition could lead to anerroneous non-linear motion movement compensation.

Apparatus and methods of the invention will become more apparent uponreview of the brief description of the drawings, the detaileddescription of the invention, and the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the objectives of the invention and other desirablecharacteristics can be obtained is explained in the followingdescription and attached drawings in which:

FIG. 1 is a schematic cross-section of a seabed seismic sensor,illustrating decomposition and composition of linear accelerometersignals in accordance with known techniques;

FIG. 2 illustrates one scheme in accordance with the invention for usingthe data obtained from an apparatus and method of the invention tocorrect seabed seismic data for non-linear motion; and

FIG. 3 illustrates a cross section of a cylindrical seabed seismicsensor positioned in a non-flat portion of a sea floor.

It is to be noted, however, that the appended drawings are not to scaleand illustrate only typical embodiments of this invention, and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiments may be possible.

All phrases, derivations, collocations and multiword expressions usedherein, in particular in the claims that follow, are expressly notlimited to nouns and verbs. It is apparent that meanings are not justexpressed by nouns and verbs or single words. Languages use a variety ofways to express content. The existence of inventive concepts and theways in which these are expressed varies in language-cultures. Forexample, many lexicalized compounds in Germanic languages are oftenexpressed as adjective-noun combinations, noun-preposition-nouncombinations or derivations in Romanic languages. The possibility toinclude phrases, derivations and collocations in the claims is essentialfor high-quality patents, making it possible to reduce expressions totheir conceptual content, and all possible conceptual combinations ofwords that are compatible with such content (either within a language oracross languages) are intended to be included in the used phrases.

The invention describes seabed seismic sensors and methods of using sameto reduce the effects of non-linear (rotational, rolling, and/orpivoting) motions of the sensors on the seismic data. The invention alsodescribes modeling one or more of these seabed seismic sensor movementsbased on historical and/or real-time data records, which may be usefulboth when the sensors lie on relatively flat sea floors and non-flat seafloors. Such records may be used for quality control purposes, as wellas for correcting the multi-component seismic measurements. While theterms “seabed” and “seafloor” are used herein, it will be understood bythose of ordinary skill in the art that the invention is not limited touse of seismic cables and sensors on a seabed or seafloor, but theapparatus and methods may be used in all types of water and liquids andall types of terra firma permanently or temporarily under water or otherliquid (such as water/oil mixtures, emulsions, chemicals, and the like),including but not limited to fresh water, oceans, seas, transitionzones, tidal zones, lagoons, lakes, rivers, bays, gulfs, channels,inlets, fjords, lochs, streams, marshes, creeks, indeed anywhere seismicdata is desired.

We can distinguish between two categories of sources of seabed seismicsensor non-linear motion: those sources unrelated to the seismic wavesbeing measured, such as water flow currents; and those related to theseismic waves being measured. In strong water flow current situations,the strongest force applied to the cable may be the lift force becauseof non-laminar water flow behind the cable. The non-laminar water flowover a cable lying on a seabed may cause both a force caused bydisturbed flow trying to push the cable out of position, and a force(Bernoulli lifting force) trying to lift the cable and seabed seismicsensors from the seafloor. The disturbed flow near the bottom of thecable near the impact zone with the oncoming current may cause vibrationand/or rotation of the cable and/or seabed seismic sensors, which isundesirable since it creates noise in signals picked up by linear motionseismic sensing units. Furthermore, frequently, but not always, thewater flow current increases further away from the seafloor, so thatupon initial lifting, the cable and seabed seismic sensors mayexperience an even greater lift force due to the even stronger waterflow current, and any increase in cable height above the seafloor maysignificantly worsen stability. As should now be clear, the seabed cableand seabed seismic sensors are frequently subject to instability in thepresence of strong water currents at or just above the seafloor. Tidalcurrents, river currents, strong underwater streams, and bad weather(waves) are but a few examples. Water currents induce vibrations thatmay be transmitted to the seabed seismic sensors. When using previouslyknown box-like seabed seismic sensors, the noise level sensed by thelinear motion seismic sensor units may increase, which may degrade dataquality and may actually stop data acquisition. Furthermore, thecoupling of the box-like seabed seismic sensor housing to the seafloormay become poor and inconsistent, which may degrade data quality. Also,any significant cable movement may require repositioning of the cableand box-like sensors, losing time and may require picking up andre-deploying the cable.

An emerging seismic wave front will have a wave length much longer thanthe seabed seismic sensor and the linear motion seismic sensing units,and should result in a pure translation movement of the seabed seismicsensor, and therefore the linear motion seismic sensor units containedtherein. However, as the coupling of the seabed seismic sensor to theseafloor is not perfect, it is well known that emerging seismic wavefronts may cause non-linear (rotation, rolling or pivoting) and lateraldisplacement of the seabed seismic sensor, and thus the linear motionseismic sensing units.

The invention provides apparatus and methods to improve the quality ofdata collected by seabed seismic sensors in the presence of forces thatmay result in non-linear motion of seabed seismic sensors, such as waterflow currents at or near the seafloor and emerging seismic wave fronts,without significantly increasing their weight. FIG. 1 illustrates aknown apparatus and method for decomposing individual linear motionseismic sensor unit signals into horizontal and vertical componentsusing two linear motion sensing elements, for example twoperpendicularly positioned accelerometers 1 and 2 that are in a radialplane of a seismic sensor 3, laying on the seabed 4. Linear motionseismic sensors 1 and 2, along with information about the statichorizontal and vertical components of the local gravity vector, arecapable of decomposing the individual accelerometer signals into ahorizontal and a vertical component using a derived angle and summingthe contributions from the two linear motion seismic sensors, as furtherexplained herein. Rather than using two MEMS sensors which measureacceleration down to the DC component, use may be made of a combinationof seismic sensors not measuring DC component (such as a moving coilgeophone or a geophone accelerometer) and associated inclinometers(which measures the DC component of acceleration. In any embodiment, themechanical properties of the cable and the linear motion seismic sensorsare not changed significantly in a way that could impact the quality ofthe data recorded.

Referring again to FIG. 1, on Earth the only static acceleration iscaused by gravity, and because gravity is always directed towards thecenter of the earth, the orientation of the linear motion seismic sensorunits with respect to the vertical can be determined. Assuming thatstatic acceleration due to gravity is g, then the orientation angle φcan be found through the equation:φ=a tan(g _(2s) /g _(1s)).  (1)

-   -   wherein:        -   g_(2s) and g_(1s) are the static components of gravity as            measured by sensors 1 and 2, respectively.            With this information and with two perpendicular linear            motion seismic sensor units (accelerometers 1 and 2) in the            radial plane of the seabed seismic sensor, the dynamic            vertical and horizontal accelerations can be measured.            Although accelerometers are used for example here, any type            of linear motion sensor may be used, or combinations of            linear motion sensors. More specifically, the dynamic            vertical and horizontal accelerations may be computed by            decomposing the individual accelerometer signals into a            horizontal and a vertical component using equation (1) and            then summing the contributions from sensors 1 and 2. As            depicted schematically in FIG. 1, accelerometer 1 measures a            signal a₁ which is the fraction a_(T)cos(φ) of the true            vertical acceleration a_(T), while accelerometer 2 measures            acceleration a₂ which is the fraction a_(T)sin(φ) of the            true acceleration a_(T). Decomposition of the measured            signals a₁ and a₂ into vertical components yields            a_(v1)=a₁cos(φ) and a_(2v)=a₂cos(φ). Substitution of a₁ and            a₂ and summing both vertical components yields equation (2):            A _(V) _(sum) =a _(T)cos(φ)cos(φ)+a _(T)sin(φ)cos(φ)=a            _(T).  (2)            Indeed, the sum in equation (2) equals the true vertical            motion, a_(T). Similarly, the true horizontal motion can be            composed from the individual horizontal sensor motions to            yield equation (3):            A _(H) _(sum) =a _(H)cos(φ)cos(φ)+a _(H)sin(φ)cos(φ)=a            _(H).  (3)

As may be appreciated from FIG. 1, if an accelerometer (or other linearmotion sensor) were positioned away from the center of the sensorassembly, this would cause an undesired seismic sensor signal as aresult of a dynamic non-linear motion. Such rotation-caused “seismic”signals may be calculated and compensated for if the exact geometry ofthe seabed seismic sensor is known. It is simpler to avoid thiscomplication by positioning the linear motion seismic sensor units inthe center of the seabed seismic sensor assembly, which may include acylindrical housing or the cable per se.

A third motion seismic sensor, which may or may not be an accelerometer,may be oriented in the remaining orthogonal direction, which is theaxial direction of the seabed seismic sensor housing, or cable per se.This third linear motion seismic sensor may be used for measuring theorthogonal (in-line) shear wave component.

In addition to the three-component linear motion seismic signaldetection, apparatus and methods of the invention may deploy an angularacceleration detector, which may be an angular accelerometer. Theangular accelerometer may be a micro-electro-mechanical (MEMS) typeaccelerometer. An example of such an accelerometer is detailed in thepublication of a sensor known under the trade designation LISIR02(L6671), available from ST Microelectronics. The specifics of the sensorare explained in ST Microelectronics Application Note 1515, “A DigitalOutput Angular Accelerometer”, F. Pasolini, February 2002, accessible onthe date of filing this application on the Internet athttp://us.st.com/stonline/books/pdf/docs/8556.pdf. As explained in theintroduction of the Application Note, the rotational accelerometer knownunder the trade designation LISIR02 is a complete rotationalaccelerometer system based on a capacitive sensor that uses MEMStechnology, and a set of accompanying electronics that produces adigital output. The device may be interfaced to external hardware usinga standard 3-wire serial interface that allows internal registers to bewritten and rotational acceleration samples to be read. The MEMSstructure may comprise a rotor and stator assembly in which capacitivevariations occur when the relative position of the rotor with respect tothe stator changes. These capacitive variations are on the order of50×10⁻¹⁸ farads. The MEMS structure also may include actuationelectrodes that allow the rotor position to be driven externally by theprocessing electronics. The electronic processing circuitry processesthe capacitive variations that occur between the MEMS rotor and stator.A “sigma-delta” architecture may be implemented that works tocontinually restore the rotor to nominal position. The control effort,or the signal that drives the rotor to nominal, represents therotational acceleration that is present at the system location. Thiscontrol effort is a binary bit stream that is decimated by theelectronics to provide a noise-reduced output. Gain and offsetadjustments are applied to the decimated bit stream to produce theacceleration samples. Accelerometer samples then are clocked into afour-deep data FIFO within the IC. The decimation and FIFO stages areclocked in a free-running manner based on the selection of either aninternal or external clock source.

The angular accelerometer may be used to measure non-linear motion ofthe seabed seismic sensor or seismic cable about any axis, which may beconverted into a measure of horizontal displacement at the circumferenceof the seabed seismic sensor. In general, an emerging wave front willcause both non-linear and lateral displacement of the seabed seismicsensor and/or seismic cable housing the linear motion seismic sensingunits. The axial component may be measured using the axial (orthogonal)sensor only, since the non-linear motion is restricted in most cases,although it is possible to measure and compensate for this also, asexplained herein. The lateral component may be calculated by adding thelateral acceleration a_(H) _(sum) (computed by composition as detailedpreviously in equation (3)) to the product of the angular accelerationand the radius of the seabed seismic sensor. Finally, the verticalcomponent a_(V) _(sum) may be obtained also by the composition asdetailed previously (equation (2)).

In some embodiments of the invention, non-linear motion may be measuredwithout using a non-linear motion sensor, but using an extra linearmotion sensor (accelerometer or other linear motion seismic sensingunit). As described previously, if an accelerometer (or other linearmotion seismic sensing unit) was positioned away from the center of theseabed seismic sensor assembly, this may cause an undesired seismicsensor signal as a result of a dynamic rotation. This may be used tocalculate the rotation: if a third accelerometer or other linear motionsensor is positioned away from the axis of the seabed seismic sensorparallel to the seabed cable longitudinal axis, we would be able topredict its translation movement by decomposing the measured signal ofthe first two accelerometers along the axis of the third one. Assumingthat the sensors are correctly calibrated, the difference between thepredicted and the measured signal corresponds to the accelerationcorresponding to the rotation of the cable/seabed seismic sensor aboutits axis, from which we can calculate the rotationalacceleration/motion. This concept may be broadened to the 3D case and torotations along any axis by having 4 or more accelerometers/linearmotion seismic sensing units.

FIG. 2 illustrates one scheme in accordance with the invention for usingthe data obtained from an apparatus and method of the invention tocorrect seabed seismic data. A vessel 8 in ocean 10 tows one or moreseismic sources, 12. A cable 6 lays on seabed 4. Cable 6 may rotate asillustrated by curved arrow R. When cable 6 rotates, pivots, and/orrolls, one or more linear motion seismic sensing units 3 (only 1 isillustrated) also rotates, pivots, or rolls. Note that it is notpossible to distinguish between a lateral movement and a seismic signal.Source 12 transmits periodic acoustic pulses 14, which ideally arereflected 16 off of a subterranean hydrocarbon deposit 18. Some of thereflected signals are received at linear motion seismic sensing units 3.Raw seismic data 20 is collected, as well as raw non-linear and linearmotion data 22. Decomposing measured signals into vertical andhorizontal components is then performed, represented at box 24, followedby composing into true vertical and horizontal motion at 26. Raw seismicdata may then be corrected, either in real time, or at another time, asrepresented at 28, to form corrected seismic data 30. Other optionalcorrection subroutines (not shown) may be integrated with the schemedepicted in FIG. 2, and those of ordinary skill in the art of marineseismic hydrocarbon exploration will understand that there are manyexisting and foreseeable equivalents to the scheme depicted in FIG. 2.

FIGS. 1 and 2 deal with non-linear motion of a seabed seismic sensor orcable under circumstances when the seabed is relatively flat. Theinvention in relation to FIGS. 1 and 2 addresses the problem ofdistorted transfer from a horizontal earth motion into a proportionalhorizontal sensor motion, as well as the undesired cross-feed fromhorizontal into vertical motion. These distortions may arise from one ormore static or dynamic non-linear motions of the seabed seismic sensor.In the case of dynamic rotation, some of the horizontal energy from theearth is transferred to rotational energy of the seabed seismic sensor.We described how one can remove this distortion from the horizontalcomponent by measuring the rotation and adding the product of theangular acceleration and the radius (or effective radius, fornon-cylindrical housings) of the seabed seismic sensor housing to thehorizontal measurement. We also described in relation to FIGS. 1 and 2how a dynamic rotation of the sensor housing may lead to cross-feed tothe vertical component if the sensor housing is not cylindrical or ifthe linear motion seismic sensing units are not placed in the geometricmiddle of the seabed seismic sensor housing.

While FIGS. 1 and 2 deal with rotation of a seabed seismic sensor orcable under circumstances when the seabed is relatively flat, of course,this is not always true. Two additional sources of distortion andcross-feed that have not been mentioned so far are if the sea floor isnot flat around the seabed seismic sensor, which is quite likely in reallife, or if the seabed seismic sensor gives some unwanted response toone or more dynamic non-linear motions, for instance due to the factthat the sensor is not a true point receiver but has a finite extension.The present invention also addresses these two additional sources ofdistortion to the horizontal and vertical components. FIG. 3 illustratesone simplified situation, illustrating a cross section of a cable orseabed seismic sensor 6 lying in a half-cylindrical cavity 40. Thisexample assumes cylindrical cable and/or seabed seismic sensor, but theinvention is not so limited. Cable or seabed seismic sensor 6 has aradius of r, while the half-cylindrical cavity 40 has a radius of R inthis simplified representation. One approach is to model the sensorresponse due to one or more non-linear motions on a non-flat sea floor.Another solution is to measure, under laboratory conditions, sensorresponse due to one or more non-linear motions, and use thesemeasurements actively in data correction schemes.

One possible model of the situation is a “ball in a bowl”, asillustrated in FIG. 3. The idea is that a seabed seismic sensor housinghaving a radius (or effective radius) of r is free to roll back andforth within the “bowl”, representing a locally curved sea floor 40having radius R. This model incorporates both a flat and a locallycurved sea floor, depending on the choice of the radius R of the “bowl.”Assuming that the earth motion is known one may compute how the seabedseismic sensor housing moves, and hence what the linear motion seismicsensing units will record. This model may also be used for inversemodeling, i.e. to find the true response of the earth given the linearseismic sensing unit response. The radius R, as well as drag andfriction coefficients, are generally unknown in this model. Suchparameters will exist in all models and may for instance be determinedby tuning the model to measured data. It is likely that these parameterswill change with area and seabed conditions, hence one will most likelyneed some measurements for any new survey, but not necessarily for everyseabed seismic sensor. Another reason for using modeled instead ofmeasured rotations is that the rotation measurement itself may be noisy.Fitting a modeled rotation record to the measured one using a local orremote calculation unit may provide a reduced noise or noise-freerotation record and may reduce or eliminate unwanted noise in seismicdata through the rotation correction scheme.

In addition to the possibility of correcting the sensor responses basedon one or more non-linear motion measurements, and modeling, one may usethe non-linear motion measurements for quality control purposes, eitherwith or without correcting the sensor responses. Reasons for doing thismay be that the correction scheme is too computationally expensive orthat the non-linear motion measurement is noisy as mentioned above,hence the correction scheme may introduce unwanted noise.

In the embodiments of FIGS. 1-3 the non-linear motion is assumed to beonly in the plane perpendicular to the longitudinal axis of the cable.Although non-linear motion is most likely in this plane, non-linearmotion, for example may occur about any axis. Providing we characterizethe seabed seismic sensor non-linear motion around any axis (as byhaving a three-axis of symmetry rotation sensor), the methods describedabove may be extended to three dimensions, and may be used to compensatefor rotations of the seabed seismic sensors around any one or more axis.

It should be noted that in the embodiments discussed in reference toFIGS. 1, 2 and 3, and indeed all embodiments and figures of the presentinvention, geophones are not depicted, but would typically be spacedalong and/or inside each seabed cable. If desired, to reduce some of thenon-linear and lateral movements of the seabed seismic sensors andcables, cleats may be used, although the number of cleats may be less inthe practice of the present invention, as non-linear and lateralmovement is not necessarily disadvantageous.

In sum, apparatus and methods of the invention may use one or morenon-linear motion sensors to measure one or more non-linear motionsabout any axis, or non-linear motion about any axis of the seabedseismic sensor may also be derived from the linear motion seismicsensing unit (geophones and/or accelerometers) measurements if more thanthree are used. The translation motion will in all embodiments be fullycharacterized by three linear motion seismic sensing units such asgeophones and accelerometers. If only a translation motion is measured,a fourth linear motion seismic sensing unit, located somewhere else inthe seabed seismic sensor housing, would record exactly the same signalthan may be obtained by rotation data from the first three linear motionsensing units in the direction of the fourth. If there is a difference,assuming that the linear motion sensing units are well calibrated, it iscoming from some type of non-linear motion.

Other than the inventive features discussed herein, seabed seismicsensors and their support cables useful in the invention include thosedescribed in the article “Shear Waves Shine Brightly”, Oilfield Review,pages 2-15 (1999), and typically comprise an instrumented cable packedwith seismic receivers and additional motion sensors, similar to thestreamers that are towed in conventional marine surveys, but designed tooperate on the seafloor. One useful seabed seismic cable for use in thepresent invention, known under the trade designation “NESSIE 4C”,contains multiple linear motion seismic sensing units each containingone hydrophone and three orthogonally oriented geophones inside thecable, distributing their weight for optimal coupling to the seafloor.These cables may be modified in accordance with the invention to includeone or more non-linear motion sensors described herein, or additionallinear motion sensors. Each cable may house hundreds of four-componentseismic sensors, and multiple motion sensors. Full particle-motionvector recording of all P (pressure) and S (shear) wave fronts may beachieved, along with the pressure wave front familiar from towedstreamers. This design was an improvement over conventional ocean bottomcables, which may be employed in the present invention as well,comprising only a hydrophone and a vertically oriented geophone strappedto the outside, and including motion sensors as described herein;however, this arrangement is incapable of recording the fullparticle-motion vector. The motion sensors would, however, compensatefor any non-optimal coupling of this type of cable to the seafloor.

Another seabed cable useful in the invention is described (except forthe inventive concepts described herein) in U.S. Pat. No. 6,021,091,also assigned to WesternGeco, which describes an elongated ocean-bottomseismic cable section of a desired length manufactured by assembling astress member in combination with a plurality of signal communicationchannels. A plurality of receiver clusters is fastened to the assemblyat desired intervals. Each cluster includes at least two multi-axial,gimbal-supported seismic receivers that are symmetrically mounted aboutthe axis of the cable assembly. Output signals from the common axes ofthe respective multi-axis receivers of each cluster are coupled witheach other through a suitable filter and linked to corresponding signalcommunication channels. The cable section is terminated by connectorsfor providing mechanical and communication linkage to other sections andeventually to signal-processing instrumentation.

The motion sensing elements (linear and non-linear) may be included withthe cable during the manufacture of the cable, or added afterwards.Methods of making motions sensors, for example MEMS sensors, are not thesubject of the invention, these methods being known from publicationssuch as U.S. Pat. Nos. 4,932,261; 5,831,164; and 6,257,062. Thesepatents discuss the benefits of motion sensors, but do not suggest theapparatus and methods of the present invention. For example, the 261patent discloses a spring-mass-support structure arranged as anaccelerometer with two electrostatic feedback loops so as to measure twocomponents of acceleration. Two of such accelerometers areadvantageously used on a submerged platform to determine the directionof a sound source in a body of water. One of such feedback loops of eachaccelerometer is used to measure low frequency gravitationalacceleration applied to the platform as a result of the tilt of theplatform due to the action of current and wave action of the water. Theother of the feedback loops of each accelerometer measures sound waveinduced acceleration on the platform. The gravitational acceleration ofeach accelerometer is used to compensate the higher frequencyacceleration for the effect of tilt of the platform. The 164 patentdiscloses a two degree of freedom (2 DOF) accelerometer comprising twoimbalanced sensing modules (i.e., bar modules having an unbalance) suchthat two sensing modules are located in the plane to be measured. Thetwo imbalanced sensing modules have force balance and are used tomeasure linear and angular accelerations in two degrees of freedom. Asingle modular design is used for both of the axes. Each of the twoindividual sensing modules has a housing containing a proof mass foreach measured output and a support of the proof mass with the supportoptimized for the sensitive axis selected for sensor output and havinghigh rigidity in all other axes of the support. Also provided iselectrostatic servo force balance technology. Aproportional-integral-derivative (PID) controller is used to obtain aclosed loop bandwidth of 1 kHz and a steady state error of zero forfull-scale accelerations up to 75 g's and 1000 rad/sec². The 062 patentdiscloses an angular accelerometer having a substrate, a plurality offixed electrodes supported on the substrate and each including fixedcapacitive plates, and a rotational inertia mass including a pluralityof movable capacitive plates arranged to provide a capacitive couplingwith the fixed capacitive plates. The rotational inertia mass isrotationally movable relative to the fixed electrodes in response toangular acceleration. The angular accelerometer further includes supportmembers for supporting the rotational inertia mass and biasing therotational inertia mass relative to the fixed electrodes duringrotational movement of the rotational inertia mass. An input is coupledto the plurality of fixed capacitive plates for receiving an inputsignal, and an output is coupled to the other of the plurality of fixedand movable capacitive plates for providing an output signal that variesas a function of the capacitive coupling between the fixed and movablecapacitive plates and is indicative of angular acceleration

Desired properties of seabed cables useful in the invention, other thanbuoyancy properties, are similar to those typically specified for towedstreamers, such as streamers described in various patents. The cable maybe water tight, armored, and carry sufficient number of wires or opticalfibers to carry acoustic data from the sensors in the cable to a datacollection unit. Methods of making cables having protruding cleats arealso known. The cables of the invention may have a minimum workingstrength of 4000 pounds and minimum breaking strength of 8000 pounds,and may have separate electrical (or fiber optic) and mechanicalterminations to provide strain relief for electrical (or fiber optic)connectors and to enable the electrical or optical connectors to beconnected and disconnected while maintaining a working strength tension.

The cable outer skin may be made of any natural or synthetic material.Useful synthetic materials include thermoplastic elastomers, such aspolyurethane thermoplastic elastomers, polyester thermoplasticelastomers, and ionomeric thermoplastic elastomers. The syntheticmaterials may have additives therein which reduce or prevent fouling ofthese elements. Thermoplastic elastomers are defined and reviewed inThermoplastic Elastomers, A Comprehensive Review, edited by N. R. Legge,G. Holden and H. E. Schroeder, Hanser Publishers, New York, 1987.Thermoplastic elastomers, upon heating above the melting temperature ofthe hard regions, form a homogeneous melt which can be processed bythermoplastic techniques (unlike elastomers), such as injection molding,extrusion, blow molding, and the like. Subsequent cooling leads again tosegregation of hard and soft regions resulting in a material havingelastomeric properties, however, which does not occur withthermoplastics. Some commercially available thermoplastic elastomersinclude segmented polyester thermoplastic elastomers, segmentedpolyurethane thermoplastic elastomers, segmented polyurethanethermoplastic elastomers blended with other thermoplastic materials,segmented polyamide thermoplastic elastomers, and ionomericthermoplastic elastomers.

Apparatus and methods of the invention may also have advantages inprotecting the cable against abrasion and cuts in case of dragging onthe seafloor, particularly when thermoplastic elastomers are used forcable outer skin. Rather than moving in a cross-line direction, thecable may be allowed to rotate slightly.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. In the claims, no clauses are intended to be inthe means-plus-function format allowed by 35 U.S.C. § 112, paragraph 6unless “means for” is explicitly recited together with an associatedfunction. “Means for” clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures.

1. An apparatus comprising: a seabed seismic sensor, the sensorcomprising at least one linear motion seismic sensing unit, and meansfor sensing one or more non-linear motions of the seabed seismic sensor.2. The apparatus of claim 1 wherein the means for sensing one or morenon-linear motions is selected from one or more rotational motionsensors, one or more linear motion sensors when three linear motionseismic sensing units are present, and one or more sensors capable ofsensing both rotational and linear motion.
 3. The apparatus of claim 2wherein the means for sensing non-linear motion is a rotational motionsensor.
 4. The apparatus of claim 3 wherein the rotational motion sensoris adapted to resolve rotational movement about a substantiallyhorizontal axis into vertical and horizontal components.
 5. Theapparatus of claim 3 wherein the rotational motion sensor is selectedfrom sensors capable of measuring acceleration in one degree ofrotational freedom and sensors capable of measuring accelerations in twoor more degrees of rotational freedom.
 6. The apparatus of claim 2wherein the seabed seismic sensor comprises three linear motion sensingunits centrally positioned in the seabed seismic sensor, and the meansfor sensing rotational motion is a fourth linear motion sensorpositioned in the seabed seismic sensor but away from the threecentrally positioned linear motion sensing units.
 7. The apparatus ofclaim 1 wherein the linear motion sensor is selected from sensorscapable of measuring acceleration in one degree of linear freedom andsensors capable of measuring accelerations in two or more degrees oflinear freedom.
 8. The apparatus of claim 1 comprising features selectedfrom: a) the at least one linear motion sensing unit comprising one ormore micro-machined-electro-mechanical sensors; b) the at least onelinear motion sensing unit comprising a three-component linear motionmicro-machined-electro-mechanical sensor; c) the three-component linearmotion micro-machined-electro-mechanical sensor is positioned inpositions selected from a center of a housing of the seabed seismicsensor and positions other than the center; d) a plurality of seabedseismic sensors spaced along a cable; and e) combinations thereof. 9.The apparatus of claim 1 comprising a seabed seismic sensor non-linearmotion database comprising data selected from modeled sensor non-linearmotion data, measured sensor non-linear motion data, and both modeledand measured sensor on-linear motion data, wherein either one or both ofthe modeled and measured sensor non-linear motion data is collected fornon-flat seabed conditions.
 10. The apparatus of claim 9 comprising acalculation unit adapted to fit modeled sensor non-linear motion data tomeasured sensor non-linear motion data when the sensor is in a non-flatportion of seabed.
 11. A method comprising: a) acquiring seismic datausing a seabed seismic sensor on a seabed; b) sensing one or morenon-linear motions of the seabed seismic sensor; and c) correcting theseismic data for the non-linear motion of the seabed seismic sensor. 12.The method of claim 11 wherein the sensing one or more non-linearmotions of the sensor is selected from sensing non-linear motion,sensing one or more linear motions, and sensing both non-linear andlinear motion.
 13. The method of claim 11 including selecting from: a)methods wherein the sensing one or more non-linear motions of the sensorcomprises sensing non-linear motion, and the correcting seismic datacomprises resolving non-linear movement of the sensor about asubstantially horizontal axis into vertical and horizontal components;and b) methods wherein the sensing the non-linear motion of the sensorcomprises sensing non-linear motion, and converting horizontal motion ofthe Earth into non-linear motion of the sensor, thereby reducing thehorizontal motion at the sensor location with respect to the motion atthe Earth-to-sensor-contact.
 14. The method of claim 11 comprisingmodeling non-linear motion of the sensor under expected environmentalconditions including a non-flat seabed, storing modeled sensornon-linear motion data, and using the data in methods selected fromquality control methods, correcting measured seismic data, andcombinations thereof.
 15. A method comprising: a) acquiring seismic datausing a seabed seismic sensor on a seabed; b) allowing the sensor tonon-linearly move about an axis of the sensor at will; c) sensing one ormore non-linear motions of the sensor; and d) correcting the seismicdata for the non-linear motion of the sensor.
 16. The method of claim 15wherein the sensing non-linear motion of the sensor comprises sensingnon-linear motion, sensing one or more linear motions, and sensing bothnon-linear motion and linear motion.
 17. The method of claim 15 whereinthe sensing non-linear motion of the sensor comprises sensing non-linearmotion, and converting horizontal motion of the Earth into non-linearmotion of the sensor, thereby reducing the horizontal motion at thesensor location with respect to the motion at theEarth-to-sensor-contact.
 18. The method of claim 15 wherein the sensingone or more non-linear motions of the sensor comprises sensing both oneor more non-linear motions and one or more linear motions.
 19. Themethod of claim 15 wherein the correcting the seismic data comprisesresolving non-linear motion of the sensor about a substantiallyhorizontal axis into vertical and horizontal components.
 20. The methodof claim 15 comprising modeling non-linear motion of the sensor underexpected environmental conditions including a non-flat seabed, storingmodeled sensor non-linear motion data, and using the data during methodsselected from quality control methods, correcting measured seismic data,and combinations thereof.